1. Field of the Invention
The present invention relates to an inkjet printhead in which heating elements for discharging ink and driving circuits for driving them are formed on a single element substrate, and a printing apparatus using the inkjet printhead.
2. Description of the Related Art
As disclosed in the U.S. Pat. No. 6,290,334, heating elements and their driving circuits on a printhead mounted in a conventional inkjet printing apparatus are formed on a single element substrate using a semiconductor process. The U.S. Patent Publication No. 2005/0206685 discloses a technique of supplying power to heating elements by a so-called constant current driving method.
FIG. 5 is a circuit diagram for explaining the arrangement of a driving control circuit arranged on the element substrate of a printhead using a conventional constant current driving method.
In FIG. 5, heating elements 101a1 to 101mx are used to print. The respective heating elements are energized to generate heat, discharging ink droplets from corresponding nozzles. The heating elements 101a1 to 101mx are divided into groups a to m. Each group includes x heating elements, and x MOS transistors arranged in correspondence with the respective heating elements. MOS transistors 102a1 to 102mx turn on/off power supply to corresponding heating elements. Each of constant current sources 103a to 103m is arranged in a corresponding group. A printing data supply circuit 104 controls the ON/OFF state of each MOS transistor 102 in accordance with printing data. A reference current circuit (IREF) 105 outputs a control signal to the constant current sources 103a to 103m via a line 110 to control constant current values made constant by the respective constant current sources. Power supply terminals 106 and 107 are connected to a power supply unit (not shown) outside the element substrate. The element substrate receives, via these power supply terminals, power for driving heating elements. Power supply lines 108 and 109 supply, from the power supply terminals 106 to 107 to the groups a to m, power for driving heating elements.
In group a, the MOS transistors 102a1 to 102ax are respectively series-connected to the corresponding heating elements 101a1 to 101ax. Each MOS transistor controls on/off of a current supply to the series-connected heating element. More specifically, the drain terminals of the MOS transistors 102a1 to 102ax are connected to the corresponding heating elements 101a1 to 101ax. The source terminals of the MOS transistors 102a1 to 102ax are commonly connected to the constant current source 103a. One end of each of the heating elements 101a1 to 101ax is also commonly connected to the power supply line 108. The MOS transistors 102a1 to 102ax are the first driving switches of the heating elements 101a1 to 101ax. The constant current source 103a is the second driving switch of the heating elements 101a1 to 101ax. This arrangement also applies to the remaining groups b to m.
The constant current sources 103a, . . . , 103m are respectively series-connected to the MOS transistors 102a1 to 102ax, . . . , 102m1 to 102mx and the heating elements 101a1 to 101ax, . . . , 101m1 to 101mx. Each of the constant current sources 103a to 103m sets a current flowing through it to a predetermined constant current value. The constant current value is adjusted by inputting a control signal from the reference current circuit 105 via the line 110.
The printing data supply circuit 104 outputs, to the gate terminals of the MOS transistors 102a1 to 102mx, a printing data signal corresponding to an image to be printed. The printing data supply circuit 104 controls switching of the MOS transistors 102a1 to 102mx. 
FIG. 6 is a circuit diagram showing an arrangement in which MOS transistors 111a to 111m replace the constant current sources 103a to 103m in FIG. 5.
The drain terminals of the MOS transistors 111a to 111m are respectively connected to the source terminals of the MOS transistors 102a1 to 102mx. The gate terminals of the MOS transistors 111a to 111m are connected to the line 110 to receive a control signal output from the reference current circuit 105.
This arrangement sets constant currents flowing through the NMOS transistors 111a to 111m to a predetermined constant current value. The constant current value is controlled by the gate voltages of the MOS transistors 111a to 111m connected to the reference current circuit 105.
For example, if the number of simultaneously driven heating elements increases, the wiring resistance in the element substrate may rise and fluctuation between wiring resistances becomes greater and greater, and the fluctuation ratio of voltages applied to respective heating elements may increase. However, this arrangement can make energy amounts applied to respective heating elements almost constant. A wiring line outside the element substrate is shared between a plurality of heating elements. Even if the voltage drop on a common wiring line changes depending on the number of simultaneously driven heating elements, energy amounts applied to respective heating elements can be made almost constant. Even if the voltage of an external power supply unit for supplying power to the printhead fluctuates, energy amounts applied to respective heating elements can be made almost constant.
In this way, the constant current driving method can make almost constant energy amounts applied to respective heating elements that vary due to various factors. This is because the reference current circuit 105 controls currents input to the gate terminals of the MOS transistors 111a to 111m serving as constant current sources. Voltage fluctuations upon variations of the ON resistances of the MOS transistors 111a to 111m can also be absorbed.
In addition to the above-described voltage fluctuations, when printheads different in the resistance value of the heating element are manufactured, power generated by the heating element may differ between the printheads. In this case, according to the constant current driving method of always supplying a constant current to heating elements, generated energy also differs between the printheads due to the difference in the resistance value of the heating element. To solve this, a D/A converter is arranged, as shown in FIG. 7, to externally input a signal and set the value of a current supplied from a constant current circuit. More specifically, in FIG. 7, current value setting data are serially input as digital signals to a D/A converter 701 via terminals 702 and 703. Based on the data, a voltage generated by the D/A converter 701 is supplied to the reference current circuit 105. This voltage is reflected in the gate voltages of the MOS transistors 111a to 111m, finally supplying a current of a desired value to respective heating elements. In this manner, the printhead performs constant current driving using an optimum set current based on the heating element resistance value acquired in advance.
Setting of a current value by the D/A converter is performed by circuits formed on a single element substrate, so the setting of the current value can be changed quickly. For this reason, even when energy applied to the heating element is to be changed during printing due to any factor, the setting of the current value can be changed. For example, when the printhead temperature changes during printing, the time of energization to the heating element is generally controlled to keep the ink discharge characteristic constant. Instead, the value of a current flowing through the heating element can be changed to keep the ink discharge characteristic constant.
However, to meet the demand for suppressing voltage fluctuations caused by a variety of factors and the demand for changing energy applied to the heating element, the range of voltage fluctuations absorbed by the MOS transistor serving as a constant current source becomes very large. Voltage fluctuations absorbed by the MOS transistor are voltage fluctuations occurred when the number of simultaneously driven heating elements increases for time-division driving or the like. More specifically, such voltage fluctuations are those absorbed by the MOS transistor to correct instantaneous fluctuations of the current/voltage upon simultaneous driving. To suppress voltage fluctuations, for example, the size of the MOS transistor needs to be increased. However, this leads to a large circuit size and high cost. A voltage absorbed by the MOS transistor is consumed as power by the ON resistance, wasting power. This leads to power loss, raises the printhead temperature, and negatively affects stable ink discharge.
If the voltage fluctuation range becomes wide, the current value setting range by the D/A converter also becomes wide. In a D/A converter with a resolution kept unchanged, the minimum fluctuation width becomes large, and the precision of the set current becomes low. To keep the minimum fluctuation width constant, the resolution needs to be increased, but the size of the D/A converter itself increases.
Further, if the range of voltage fluctuations absorbed by the MOS transistor becomes wide, a voltage applied between the power supply and GND rises in proportion to the range because a voltage applied to the heating element is always constant and the heating element and MOS transistor are series-connected to each other. If the voltage rises excessively, it exceeds the tolerable level of the MOS transistor. To increase the tolerable level of the MOS transistor, the transistor manufacturing process needs to be complicated. In addition, the transistor size increases, and the cost rises.